1. Field of the Invention
The present invention relates to a stain-proofing agent (i.e. an agent which enables release of soils and stains by the action of water and the like) used for stain-proofing treatment of the surfaces, for example, of wood fiber cement boards, calcium silicate boards, cement (concrete) boards, metal plates or boards, or glass plates or boards, as well as to a building board treated with the stain-proofing agent.
2. Description of the Prior Art
Architectural substrate boards such as, for example, external wall materials such as siding boards are generally coated with a coating composition on their surfaces and are applied with a stain-proofing agent which forms a stain-proofing film having a self-cleaning function to remove stains adhered to the surfaces after attachment.
As this kind of stain-proofing agent has been used such an agent which forms a super hydrophilic stain-proofing film on the surface to be treated. Upon application of the stain-proofing agent onto the surface of a substrate, a super hydrophilic stain-proofing film is formed thereon. When stains are attached to the surface of the substrate, water applied to the surface is absorbed by the super hydrophilic stain-proofing film and, as a result, the stains float on the water and are washed away together with the water (i.e. self-cleaning effect).
In order to form a super hydrophilic stain-proofing film on the surface of a substrate, mainly an aqueous dispersion of silica fine particles (colloidal silica) has been hitherto used.
For example, Japanese Laid-open Patent Publication No. 6-71219 gazette (JP 6-71219 A) discloses a method for forming a stain-proofing film which comprises applying an aqueous dispersion of colloidal silica having an average particle diameter of not more than 100 nm to a coat of an aqueous emulsion of a synthetic resin to form a film of colloidal silica on the surface of the coat.
Japanese Laid-open Patent Publication No. 2002-338943 gazette (JP 2002-338943 A) discloses a method for forming a stain-proofing layer which comprises applying a liquid containing colloidal silica and an alumina/aluminum-magnesium composite oxide for providing water-proof and alkali-proof properties to the coated surface.
The above-described silica fine particles give super hydrophilicity to the treated surface of the substrate owing to the presence of silanol group on the surface of the particles.